Learn Icelandic: Klippingin er eftir þrjá daga.

When eftir is followed by an amount of time in the accusative case, it acts as a future time anchor. It translates to "in" that amount of time in English, telling us exactly when a future event will happen.

I thought the preposition eftir took the dative case. Why is þrjá daga in the accusative?

That is normally true! When eftir means "after" an event or physically "behind" something, it takes the dative. But there is a special rule for time: when eftir is used to mean "in [amount of time]" pointing to the future, it always forces the time words into the accusative case.

Why is the number three written as þrjá instead of the base form þrír?

In Icelandic, the numbers 1 through 4 must agree with the noun they describe in gender, number, and case. Since dagur (day) is a masculine noun, and eftir forces it into the accusative plural form daga, the number must also be in its masculine accusative form, which is þrjá.

How is the word klippingin built?

It is the base feminine noun klipping (haircut) with the attached definite article -in at the end. Because it is the subject of the sentence, it is in the nominative case.